Solvent reorganization energies measured by an electron transfer reaction in supercritical ethane

Abstract

The intermolecular electron transfer reaction between a biphenyl anion and pyrene in supercntical ethane was studied using pulse radiolysis. Second-order electron trausfer rates were found to be of the order of 1011M-1 s-l. The rate constants appear to be approximately constant over the pressure range 55-133 bar. Two possibilities are discussed that could explain the present results: solvent clustering; or a dependence of the solvent reorganization energy on pressure. Reorganization energies Er of non-polar supercritical ethane were estimated from the observed rate constant using the modified Marcus equation. Er may be larger than normally expected for nonpolar solvents because of density fluctuations. .-Introduction It is well known that density fluctuations [1] and clustering of solvent molecules [2][3][4][5][6][7][8] are observed in supercritical fluids(SCFs). The local density of SCFS around a solute may be significantly greater than the bulk density of fluid not only in polar fluids [9] but also in non-polar fluids [ 10]. The number of molecules around a solute is sibtificantly changed near the critical pressure [11]. RecentIy,Matyushov and Schrnid[12] have theoretically studied solvent reorganization energy Er arising from charge separation and recombination reactions in non-polar fluids. According to their results, the Er in non-polar solvent is not zero but can be represented as the sum of two terms that arise from liquid polarization and density reorganization, with the latter component being of much greater importance.For this reason we felt that electron transfer (ET) reactions in non-polar SCFS might show a new aspect of the solvent reorganization energy in non-polar fluids. While radical reactions have been studied in SCFS[13-16 for example], we are unaware of any ET reactions that have been studied in SCFS.In this work we present the intermolecular ET reactions in non-polar SCFS. We chose to use ethane as a non-polar solvent because it was relatively unreactive to the ions formed and a supercritical fluid could be formed conveniently. Biphenyl anion and pyrene were used as au electron donor and acceptor, respectively. The solvent reorganization energies were estimated using the measured ET rates. Materials and MethodsHi~h messure eauir)ment Experiments were run with a cylindrical stainless-steel high pressure cell (Takagi equipment CO., LTD., 4 cm in O.D., 9.5 cm in length). The 1 cm thick supra.silwindows were mounted to the cell using Teflon o-rings. The optical path length is 6 cm and the cell capacity is 4.2 cm3. The temperature of the cell was maintained constant to~0.1'C at 35°C (A 0.5"C) using an Omega temperature controller (Model CNIOOIRTD), a cartridge heater and a platinum resistance thermometer. The thermometer was installed into the cell so that it is in contact with the fluid. Pressures were generated using a JASCO HPLC pump (Model PU-980, with a flow controller and pressure sensor). The fluid pressure was monitored with Cole-Parrner digital pressure meter (Model 7350-38) and pressu...

Charged Species in the Radiolysis of Supercritical CO2

“…Pressure in the cell was adjusted using a HPLC (JASCO, model PU-980) pump and monitored with a Cole-Parmer digital pressure meter (model 7350-38). The experimental arrangements were similar to those described previously. , …”

“…The local solvent density of supercritical fluid may be significantly greater than the bulk density of fluid. Unusual microscopic solvation of solutes (sometimes referred to as the clustering of solvent molecules around the solute molecule) in these systems affects the kinetics of energy and charge-transfer reactions. − Fast techniques of flash photolysis , and pulse radiolysis , have been employed to examine both radical and electron-transfer reactions to determine the effects of solvation and solvent reorganization energies.…”

7 Radiation chemistry

“…Two possible explanations are discussed among which is a solvent reorganization energy of approximately 0.35 eV dependent on pressure, a ¢gure higher than expected due to density £uctuations. 177 Inorganic and small organic solutes…”